Steve SpanglerYouTube was only five months old when this former teacher first taught viewers how to turn a bottle of Diet Coke and a roll of Mentos into an exploding geyser. Now a full-time YouTube teacher, Steve Spangler makes science fun with "The Spangler Effect."

Rob TarrouWhat started off as a way to help students at St. Pete High School has now gone worldwide. Rob Tarrou began filming his energetic math lessons from his house with his wife behind the camera. Those videos have now been seen hundreds of thousands of times, from New York, to Hungary, to Israel.

Alex DainisAlex Dainis is "a biology nerd, music lover, film geek.” She gets the ideas for her YouTube videos from conversations she has with her friends, such as: "Why do we get brain freezes when we eat ice cream too fast?" or "Why do we sneeze?"

Paul AndersonPaul Anderson started creating videos for the students in his class several years ago. A friend encouraged him to start uploading them to YouTube, and he says this has brought a whole new virtual classroom of students into his life.

Keith HughesPublic school teacher Keith Hughes, who began using video in his social studies classroom in 2002, now has more than 100 videos on YouTube covering topics in U.S. history and government, political science and world history.

Kristen WilliamsKristen Williams realized that there was an entire YouTube education community when she came across the Vlogbrothers, and got hooked.

Brothers John and Hank GreenJohn Green, YouTube sensation and New York Times bestselling author, has a quirky, fast-paced teaching style as he leads a Crash Course on The Fall of Rome, The Dark Ages and The Mongol Empire. His brother Hank Green explains topics in chemistry, biology, and ecology to name a few.

Rachel SmithNew Yorker Rachel Smith is a classically trained opera singer and linguist who now teaches ESL on YouTube as a career. Rachel is focused specifically on pronunciation. She has adoring students across the globe.